DESCRIPTION

A soft token is a lock which is only held while a thread is running. If
a thread explicitly blocks, all its tokens are released, and reacquired
when the thread resumes. While a thread blocks, the conditions protected
by a soft token may change and may need to be reevaluated on wakeup.
Tokens may be taken recursively. However, tokens must be released in the
reverse order they were acquired.
Tokens may be acquired in shared mode, allowing multiple concurrent hold-
ers, via lwkt_gettoken_shared(), or in exclusive mode, allowing only one
holder, via lwkt_gettoken(). It is safe to acquire a token shared while
holding it exclusively. A thread attempting to acquire a token exclu-
sively after holding it shared will deadlock.
The pool token interface exists to allow using tokens with data struc-
tures which may be deallocated. It allows getting a token reference from
an address, which is implemented by a set of statically allocated tokens
and a hash function.
It is not recommended to take pool tokens in shared mode. A hash colli-
sion from a subsequent exclusive pool token request will hit the exclu-
sive-after-shared deadlock.
The lwkt_token_init() function is called to initialize a token. The desc
argument specifies the wait string displayed when waiting for the token.
The lwkt_token_uninit() function is called to de-initialize one. Before
using a token, it must be initialized.
The lwkt_gettoken() function attempts to acquire a token. If it is
unsuccessful, the calling thread blocks. The lwkt_trytoken() does the
same thing; however, if it cannot acquire the token, it returns 0 instead
of blocking. The lwkt_reltoken() function releases a previously acquired
soft token.
The lwkt_token_pool_lookup() function takes an address and maps it to one
of a number of statically allocated tokens. The lwkt_getpooltoken()
function acquires a token associated with an address. Use these two
functions when tokens must protect a data structure, but the structure
can be deallocated. Pool tokens do not need to be initialized.
The lwkt_token_swap() function swaps the two most recently acquired
tokens; this allows release of tokens out-of-order. This function should
not be called when less than two tokens are held.

FILES

The LWKT Token implementation is in /sys/kern/lwkt_token.c.

EXAMPLES

A simple example of using a token to protect access to a data structure:
/* Data structure to be protected */
struct protected_data {
struct lwkt_token tok;
int data;
};
struct protected_data pdata;
/* Called early in boot */
void
init(void)
{
lwkt_token_init(&pdata.tok, "example");
pdata.data = 0;
}
/*
* A silly kthread; it uses a token to protect pdata.data.
*/
void
kthread1(void)
{
int local;
/*
* Get the soft token.
*/
lwkt_gettoken(&pdata.tok);
for (;;) {
local = pdata.data++;
tsleep(pdata, 0, "sleep", 0);
/*
* While we are asleep, we do not hold the token. When we
* awake here, we will hold the token again, but we may not
* depend on local reflecting pdata.data.
*/
local = pdata.data;
if (local == 4)
break;
}
/*
* Release the token.
*/
lwkt_reltoken(&pdata.tok);
}
An example using pool tokens:
struct dynamic_data {
int ref;
};
/*
* Use a token to protect a reference count in a dynamic structure.
* Embedding a token in the structure would be inappropriate, since
* another thread may attempt to take the token after we have freed
* the object but before we have removed all external references to it.
*/
void
kfunction(struct dynamic_data *dynptr)
{
struct lwkt_token *tok;
/*
* Get a token from the associated with the address of dynptr
*/
tok = lwkt_getpooltoken(dynptr);
dynptr->ref--;
if (dynptr->ref == 0)
free(dynptr);
/*
* Release the token via its reference, as above
*/
lwkt_reltoken(tok);
}

NOTES

Soft tokens are not released when a thread is preempted; they are only
released when a thread explicitly blocks, such as via tsleep() or
lwkt_switch().
If lwkt_gettoken() blocks while attempting to acquire a token, all cur-
rently-held tokens will be released till a thread can acquire all of them
again.
When tokens are held and tsleep_interlock() is used, tokens are not
released until blocking happens - that is until the tsleep() paired with
the tsleep_interlock() is called.